Fix PD120 SSTV decode hang and false leader tone detection

Fix infinite CPU spin in PD120 decoding caused by a 1-sample rounding
mismatch between line_samples (24407) and the sum of sub-component
samples (24408). The feed() while loop would re-enter _decode_line()
endlessly when the buffer was too short by 1 sample. Added a stall
guard that breaks the loop when no progress is made.

Fix false "leader tone detected" in the signal monitor by requiring
the detected tone to dominate the other tone by 2x, matching the
approach already used by the VIS detector.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

utils/sstv/image_decoder.py

@@ -138,9 +138,16 @@ class SSTVImageDecoder:
 
         self._buffer = np.concatenate([self._buffer, samples])
 
-        # Process complete lines
+        # Process complete lines.
+        # Guard against stalls: if _decode_line() cannot consume data
+        # (e.g. sub-component samples exceed line_samples due to rounding),
+        # break out and wait for more audio.
         while not self._complete and len(self._buffer) >= self._line_samples:
+            prev_line = self._current_line
+            prev_len = len(self._buffer)
             self._decode_line()
+            if self._current_line == prev_line and len(self._buffer) == prev_len:
+                break  # No progress — need more data
 
         # Prevent unbounded buffer growth - keep at most 2 lines worth
         max_buffer = self._line_samples * 2